It's an I-phone app from ...
Unfortunately, while the app might be great, the same cannot be said for the iPhone itself. That's a cheap little directional mic, plus a cheap little low quality pre-amp, plus a cheap little low resolution digitizer circuit. It was never meant for accurately capturing the full spectrum of low and mid frequency sounds: it was just mean to be a telephone. Proper sound level meters have good quality omni mics, good pre-amps, and good digitizers, specifically designed for the purpose of doing nothing else but measuring sound levels. When you think that even a cheap Behringer measurement mic costs upwards of US$ 70, you realize that there's no way the US$ 2.00 (guessing!) mic in an iPhone can give accurate readings.
That's probably why those numbers look so suspect. I don't doubt that you are getting good isolation from what you have done so far, but those numbers do not look like what you'd expect, if measured properly. I honestly doubt that the mic and circuit on an iPhone can even measure anything at all about 60 Hz tones: It's hard enough to do that accurately in fully equipped acoustic test laboratory, with professional equipment costing tens of thousands of dollars.
A couple of months back I did some acoustic analysis for a church in a city a few hundred miles north of here, and the Pastor also brought out his iPhone with an app, to compare against what I was using: not even close. Not even in the same ball park in the low end. His readings were off by well over 10 dB, and all over the place, as compared to what I was getting on two different systems, that agreed with each other to within about 3 dB.
I would never use a cell phone with an app for any kind of serious acoustic measurements. There is one company I heard of that makes a mic and pre-amp that can be connected to an iPhone, and used with their app for good acoustic work, but the cost of that is way more than the cost of a simple dedicated meter.
Any thought's on what might be a better option for dedicated SPL meter?
Take a look on e-bay and Amazon: you can find good new ones with the basic features for around US$ 100 and upwards. Nice ones with fancy features are around US$ 500, and real top-end ones are in the thousands of dollars, but you don't need anything that fancy. Just a little hand-held one that meets the standards and has both "A" and "C" weighting, with fast and slow response, should be good. Don't go for the very cheap "toy" ones that are appearing out of China lately, for US$ 15 to US$ 50: those are no better than an iPhone, and some aren't even that good! Get an Extech, Pyle, Phonic, Quest, Mastech, or even a good Radio Shack meter, or something like that. You can often find used ones for around US$ 75, give or take.
It would be interesting to compare those iPhone readings against the readings from a proper well calibrated meter.
- Stuart -
